Sorting through atrazine-resistant Palmer pigweed

In my last blog, I reported on the recent discovery of atrazine-resistant (AR) Palmer amaranth in Berrien County. After a more thorough investigation by Berrien County Agent Eddie Beasley, it was discovered that the corn rotation in this field was shorter than expected (Figure 1).

Because this rotation is different from the fields where AR Palmer was first confirmed in Georgia, there is some concern about how the resistance occurred.

Here are five things that you might want to consider before thinking that the sky has fallen:

#1: AR is not a brand new phenomenon. The first (ever) reported case of herbicide resistance was AR groundsel (Senecio vulgaris) way back in 1970 in the Pacific Northwest.

#2: AR-Palmer was already here. It was first confirmed in Georgia in 2008 in Macon County. Thus, the UGA Weed Group has had 6 years to work on this problem. As indicated in the previous blog, numerous other herbicides are available for use in field corn that will provide outstanding pigweed control).

However, atrazine is still very much needed in our corn weed control programs for troublesome weeds, such as sicklepod and morningglory.

#3: It is possible that AR-Palmer evolved in this field via pollen movement or physical seed movement (birds, equipment, animals, gnomes, etc.).

#4: It might also be possible that AR-Palmer evolved in this field due to the fact that the resistance is metabolism-based rather than target-site based. It has been documented in other weed species with metabolism-based resistance that cross-resistance can occur in the absence of selection pressure.

This has been confirmed in some ACC-ase resistant rigid ryegrass (Lolium rigidum) populations that were also cross-resistant to ALS herbicides that were never used in the field. Mother Nature at its finest (i.e. genetics and evolution)!

#5. Herbicide resistance is a complicated issue for sure. Consequently, it is very important for our growers to be proactive and make every effort to delay the evolution of herbicide resistance in all of our crop production systems (tillage, cover crops, crop rotations, mode of action rotations, hand-weeding, soil seed-bank management, etc.).